The Environmental Protection Agency (EPA) has issued increasingly stringent standards for particulate and NOx emissions. For example, the standards in place in October, 2002 include 0.1 g/hp-hr for particulates and 2.0 g/hp-hr for NOx. In 2007 these will be further reduced to 0.01 g/hp-hr for particulates and 0.2 g/hp-hr for NOx. Industry has intensive programs aimed at achieving these requirements.
U.S. patent application Ser. No. 10/846,780, now U.S. Pat. No. 7,269,942, discloses a method and apparatus for filtering or trapping particulate from engine exhaust and periodically disposing of the collected soot and ash. The system uses a monolithic ceramic trap having passages with porous walls through which the exhaust is passed to filter out the particulates at very high (90-97%) trapping efficiency. The systems use wall-flow traps in single or multi-trap configurations. Each of these systems can be used with any diesel engine and is capable of achieving the EPA particulate standards for the foreseeable future. Engine manufacturers can concentrate on achieving the very challenging NOx standards without concern for particulate emissions control. The particulate trap system can also be used for retrofit applications.
The wall-flow particulate trap systems use cordierite traps, such as Corning EX-80 or RC-200, to filter the exhaust gas by passing it through the porous walls of trap channels. This action removes 90-98% of the particulate and this collects on the inside surfaces of the passages as a layer or cake which after a few hours of operation increases the engine backpressure and must be removed to prevent adverse affect on engine performance. Most prior art trap systems remove this layer by burning the particulate or soot in the trap. To avoid excessive temperatures during this operation, expensive noble metal catalytic coatings are required and ultra low sulfur fuel must be used which will not be broadly available for a number of years. Also, the engines must be operated at a relatively high average load factor to assure that burn-out occurs before too much soot is collected. To assure that light-off temperatures are reached heaters such as burners or late injection coupled with catalysts are increasingly employed. Finally, the incombustible ash builds up and the traps and must then be cleaned in an expensive and disruptive maintenance operation. It is desirable to overcome one or more of these problems.